A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate (e.g. a wafer). Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a wafer (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single wafer will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the wafer parallel or anti parallel to this direction.
When projecting an image onto a wafer it is desirable to ensure that a wafer held on a wafer table is correctly positioned to receive the projected image. The wafer table is positioned using a positioning system which has six degrees of freedom (X, Y, Z, RX, RY, RZ). For any given position of the wafer table an error in each of the six degrees of freedom will be present. A calibration of the positioning system is performed to measure and record these errors. This calibration allows the wafer table to be accurately positioned during subsequent operation of the lithographic apparatus. One known method of calibrating the positioning of the wafer table is to image alignment marks onto a wafer in a closely packed arrangement, and then to develop the imaged alignment marks and measure their positions. This method is very time consuming, and may for example require several hours.
It is desirable to provide, for example, a method which obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.